I was trying to add a Sonos Boost which had become disconnected from my system.  I reset the boost numerous times following the proper procedures.  Searching on the web, I found several references to simply “Add a Boost.”  But what no one clarified is that this option doesn’t automatically appear.   The Sonos Boost must first be correctly connected to the router, in my case a router extender, Orbi.

The Sonos app on my iPhone only displayed the “Add a Boost” box option from Settings...System… “once” I connected the Sonos Boost directly to the router extender hardware that provides a WiFi mesh.  Connecting the Sonos Boost directly to the main router apparently was an incorrect configuration.  

If you’re not seeing the option, try checking your connection to the router.